Methods of producing large steel ingots

ABSTRACT

A method is provided for producing large ingots by the steps of casting an ingot, forming an axial hole through the ingot and progressively melting and refining the central zone of the ingot at the axial hole by electroslag remelting technique and progressively resolidifying the metal in the hole to form a solid ingot.

United States Patent Cooper METHODS OF PRODUCING LARGE STEEL INGOTS [75]Inventor: Lloyd R. Cooper, Pittsburgh. Pa.

[73] Assignee: Heppenstall Company, Pittsburgh,

[22] Filed: Oct. 9, 1973 [21] Appl. No.: 404,246

52 us. Cl. 164/52 511 7 1m. (1 B22D 27 02 [58] Field of Search 164/52,252; 29/5265 [56] References Cited UNITED STATES PATENTS 3 603374 9/1971Cooper 164/52 Sept. 30, 1975 3,610.318 l0/l97l Simmons 164/252 PrimaryE.\'aminerFrancis S. Husar Assistant E.\'um'iner.lohn E. RoethelAttorney. Agent, or Firm-Buell, Blenko & Ziesenhcim [57] ABSTRACT Amethod is provided for producing large ingots by the steps of casting aningot, forming an axial hole through the ingot and progressively meltingand refining the central zone of the ingot at the axial hole byelectroslag remelting technique and progressively resolidifying themetal in the hole to form a solid ingot.

4 Claims, 3 Drawing Figures U.S. Patent Sept. 30,1975

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This invention relates to new methods of producing large steel ingotsfree of central voids and cavities and particularly to methods ofproducing large steel ingots with refined central zones free fromexcessive heterogeniety, pipes, cavities and .shriiikagefvfoids that areindigenous to teemed and solidified steelingots of largev crosssections.

In the industrial production of large steel ingots, the liquid steel isteemed into a mold (usually of cast iron). in the case of fullydeoxidized (killed) steel, a reservoir of additional liquid metal, in 'arefractory lined, or

otherwise insulated shell, is provided above the main body of the ingot,to retain sufficient steel in a liquid state while the ingot body isbecoming solidified, so the liquid steel in the reservoir may feed intothe ingot body and satisfy the volume change incurred by the,

shrinkage of the solidifying steel.

The purpose of the reservoir, or sinkhead, or hot-top, is to preventpipe or'shrinkage cavities or voids within the ingot body, and to reducethe heterogeneous characteristics in the inner portion of the ingotbody. Unfortunatel y, this purpose is not always served, particularly"in those large, massive, steelin gots that may still be partiallyliquid in the central portion after two, three or even four days. Thenatural laws of selective freezing of steels dictate that some of thenamed defects and heterogenities will occur.

The invention described here makes use of the well known electroslagremelting and refining process to progressively 'remelt, refine, andresolidify the major central zone of the conventionally teemedandsolidithe necessary metal to complete the remelting and refining processherein described. The upper extension of the mold may berefractory-lined, as with a conventional sinkhead, although for theexecution of this process the upper extention may also be cast-iron; Inany event the upper extension should be cylindrical, without flutes orcorrugations, so that it may fit within a jacket as describedhereinafter.

After the ingot is solid, a round central axial hole is formed, as byforge punching, or trepanning, or by boring or trepanning on a lathe, orby other convenient means. The size of this axial hole will normally beabout per cent of the diameter of the ingot body, although larger orsmaller diameters may be used provided there is enough weight of metalavailable to fill the axial hole, and provided there is sufficient roomin which to operate the process.

The resulting ingot with the central axial hole is then positioned in anelectroslag remelting fixture, on a stool which is preferably watercooled. The stool may contain a recess or pocket which contains thestarting materials for-melting the slag and a portion of the slag to beused in the melting process. The diameter of the recess should be largeenough to permit melting a major cross section of the ingot body in theensuing process. A water cooled jacket is attached over the topextensionand shoulder of the ingot and sealed at the shoulde'r or ingotbody to prevent leakage of slag and metal as the process proceeds.

An electrode is located in the center of the axial hole. This may be aconsumable electrode of a chemical composition common to that of theingot, or as hereaft er described, the electrode may be non-consumable.The elec,trode, if non-consumable, should be watercooled, preferablymade of copper, with a water-cooled head, faced with tungsten,molybdenum, or other electricallyconductive material that will resistattack by the liquid slag.

g The electroslag melting process is started and the slag in the pocketof the stool is melted and thelwalls of the axial hole are melted andflow into the recessed stool.

,Thisraises the slag level, progressively melting more steel from theingot walls around' the axial hole. The melting is continued with theslag rising until it melts most if not all of the metal in the upperbody extension and all of the displaced metal from the ingot walls andextension has satisfied the needs of the axial hole and all of the ingotbody has solidified with a remelted and refined central zone. i

Carefully weighed amounts of slag must be used during this process, sothat the volume of slag is always known (initially and-after anyadditions). From the volume of slag, and the thickness of the slaglayer, which can be measured while the melting is in process, it ispossible to calculate the penetration of the slag into the ingot body atall times, and thereby the diameter ofthe central zone that is beingremelted and refined. Experimental measurements of this type ofremelting have shown that the outer curve, where the liquid slagcontacts the ingot wall, is most nearly represented by an ellipse, andthat the volume, V of the slag maybe determined by the equation:

rrpr 2 where r is the radius of the axial hole, I is the depth of'slag,'V isthe volume of slag and p is the penetration of melted zone.The electrical power can be increased or decreased, and the rate ofraising the electrode controlled, to produce the desired depth of slag,and thereby the desired size of the remelted and refined central zone.

This process has a significant advantage over the well-known process ofelectro slag remelting in a watercooled crucible. In the conventionalelectroslag remelting process, particularly in the large ingot sizes,there is considerable absorption of hydrogen from the atmosphere,through the unprotected surface of the liquid slag. Here, however, thisnew process is self-shielding from the absorption of hydrogen. The onlysurface of slag that is not covered is the annular space between theelectrode, and wall of the axial hole in the ingot. This surface,usually less than 20 per cent and often as low as 10 per cent of thecross-sectional area of the remelted central zone, is actually notinfluenced by the atmosphere until it approaches the top of the ingot.When the process is nearly complete, and the slag surface opens to theatmosphere, there is only a limited opportunity for absorption ofhydrogen, in the upper end of the ingot. Fortunately, because ofprogressive resolidification, the metal that is liquid at this time doesnot extend deeply into the ingot, and any hydrogen that is absorbed, hasthe opportunity to diffuse from the steel in subsequent processing.

In the foregoing general explanation of my invention, I have set outcertain objects, purposes and advantages of this invention. Otherobjects, purposes and advantages will be apparent from the followingdescription and accompanying drawings in which:

FIG. 1 is a vertical section through an ingot, stool and jacket at thestart of remelting according to this invention.

FIG. 2 is a vertical section of the ingot, stool and jacket of FIG. 1partway through the melting process of this invention; and

FIG. 3 is a vertical section of the ingot, stool and jacket oncompletion of the remelt process of this invention.

Referring to the drawings, I have illustrated an ingot having an upperbody extension 11 and a central axial hole 12 from which metal has beenremoved by any known process. The ingot is seated on a watercooled stool13 having a recess or pocket 14 beneath the axial hole 12. Startingchips and slag 15 are placed in recess 14 beneath axial hole 12 and awater-cooled copper electrode 16 is inserted in the hole 12 to contactthe starting mix 15. A water-cooled jacket 17 is placed around the bodyextension 11 and the upper ingot 10 and sealed to the ingot with asealing material 18.

The reference line AA indicates the intended top surface of the ingotafter completion of the electroslag remelting process. In the originalteeming and solidification of the ingot, allowance must be provided formore than sufficient metal above this reference line to completely fillthe central axial hole, and the pocket in the recessed stool. The weightof this upper extension of the ingot body will thus be governed by theplanned size of the axial hole and stool recess.

The process is started, the slag is first melted, and the heated slag inturn melts steel from the side wall of the ingot 10 within axial hole12. The liquid steel flows into the recessed stool, raising the slaglevel, and progressively melts more steel from the ingot wall around theaxial hole.

As the metal melts from the ingot wall, it raises the level of slag andforms a liquid metal pool 20 beneath the molten slag 21 (See FIG. 2)within an enlarged void 23 around the axial hole 12. As the slag risesthe metal at the bottom of the pool solidifies in the arc identified asS in FIG. 2. This melting and solidification progresses upwardly untilit melts most if not all of the upper body extension 11 and the axialhole 12 has been filled with remelted metal 24 and the ingot top iscovered with slag 21 as shown in FIG. 3.

The resulting ingot is free from voids, shrinkage cavities and theundesirable heterogeniety characteristics of conventional large ingots.

In the foregoing specification I have set out certain preferredpractices and embodiments of my invention, however, it will beunderstood that this invention may be otherwise embodied within thescope of the following claims.

I claim:

1. The method of producing large steel ingots with a refined centralzone comprising the steps of:

a. Casting a steel ingot of the required final size having an excess ofmetal over that finally required;

b. Forming an axial central cavity through the full length of the ingot,having a minor cross sectional area of the ingot;

c. Progressively melting and refining the central zone of the ingotaround the central axial hole by passing an electrical current through afused refining slag in the central hole by way of an electrodepositioned within the central axial hole to the ingot to melt a majorcross sectional area of the ingot, and wherein the electrode is notconsumed during the remelting process, and the metal to complete thefilling of the axial central hole is obtained totally from the initialingot; and

d. Progressively resolidifying the melted metal within the central zoneto form a solid ingot mass.

2. The method of claim 1 wherein the steel ingot is provided with a mainbody and a top extension of smaller cross section than the main body,said top extension containing sufficient metal to completely fill theaxial cavity when melted.

3. The method of claim 2 wherein said top extension is free fromexternal flutes or corrugations.

4. The method of claim 2 wherein the top extension is surrounded by acooling jacket during the melting operation.

1. The method of producing large steel ingots with a refined centralzone comprising the steps of: a. Casting a steel ingot of the requiredfinal size having an excess of metal over that finally required; b.Forming an axial central cavity through the full length of the ingot,having a minor cross sectional area of the ingot; c. Progressivelymelting and refining the central zone of the ingot around the centralaxial hole by passing an electrical current through a fused refiningslag in the central hole by way of an electrode positioned within thecentral axial hole to the ingot to melt a major cross sectional area ofthe ingot, and wherein the electrode is not consumed during theremelting process, and the metal to complete the filling of the axialcentral hole is obtained totally from the initial ingot; and d.Progressively resolidifying the melted metal within the central zone toform a solid ingot mass.
 2. The method of claim 1 wherein the steelingot is provided with a main body and a top extension of smaller crosssection than the main body, said top extension contaIning sufficientmetal to completely fill the axial cavity when melted.
 3. The method ofclaim 2 wherein said top extension is free from external flutes orcorrugations.
 4. The method of claim 2 wherein the top extension issurrounded by a cooling jacket during the melting operation.